The invention concerns a needle print head including a plurality of hinged armature magnets arranged in a ring about a needle guide with each magnet having a U-shaped magnet yoke, a magnet spool surrounding one of the U-legs of the magnet yoke and an at least nearly radially directed hinged armature, which armature at its radially inner end is rigidly connected to a print needle and is biased by spring force to a rest position away from the yoke at which it engages a stop.
In one needle print head of the previously described type, as known from DE-OS No. 27 17 007, the hinged armature is suspended at its radially outer end from a spring hanger and lays on the radially outer yoke leg so as to be pivotal on the radially outer edge of the yoke leg surface which faces it. A first disadvantage of this arrangement is that the yoke leg and the hinged armature which engages it, upon the movement of the armature, rub against one another so that a relatively strong and indefinite wear of the coengaging surfaces occurs. As a result of this, after a long operating time the quality of the print made by such a needle print head changes. According to DE-OS No. 29 43 440 an attempt has already been made to avoid this disadvantage in that the hinged armature does not pivot on an edge of the magnet yoke but instead rolls on a cylindrical pin. However, in practice it is found that in this case a pure rolling motion does not occur since the spring hanger, from which the hinged armature is suspended, causes also a slight translational movement of the hinged armature. In the case of both of these known hinged armature magnets the hinged armatures are moreover formed as two armed levers with a lever ratio of about 1:20. This large lever ratio means that no only the forces, but also the mechanical tolerances in the area of both lever arms or at the two ends of the lever arms stand in the same ratio to one another. A wear of a few one-hundredths of millimeters in the area of the armature support results in a considerably larger change in displacement at the end of the armature carrying the print needle. Hence, even with new hinged armature magnets considerable variation in the striking force with which the print needle operates on the print carrier can appear.
A further hinged armature magnet for a printer is known from U.S. Pat. No. 4,202,638 in which the hinged armature at its end opposite from the print end is fastened to a leaf spring. The hinged armature in this case lies in its rest position on the magnet yoke so that the hinged armature under the effect of the leaf spring strikes onto the print carrier. With this arrangement indeed no abrasion appears in the support area of the hinged armature, however the support axis of the hinged armature is not well defined. Besides the striking force achieved through the spring is insufficient in the case of multiple-layered print-through forms to achieve a printing of good quality on the lowermost sheet.
In a hinged armature for a needle print head known from GB-A No. 2,035,905 the hinged armature is supported on a yoke leg about a fixed pivot axis with a return spring engaging its short lever arm remote from the print needle. Because of this unfavorable lever ratio the hinged armature likewise has the above discussed disadvantages. Moreover, in this arrangement the hinged armature is not rigidly connected with the print needle but instead strikes onto the print needle which is biased to its rest position by its own return spring. Since the print needle on one hand and the hinged armature on the other hand represent two oscillating systems, which do not have oscillations of identical size, the two oscillating systems can at some drive frequencies come to a distinctly different oscillating relationship. This can lead on one hand to an increased wear of the print needle and of the hinged armature and on the other hand can lead to the creation of shadow pictures.
The invention has as its object the provision of a needle print head of the previously described type which is simple to manufacture and to assemble, which has a long service life, which makes possible a high drive frequency and which also in the case of print-through forms guarantees a clean printed picture of all layers of the form without the creation of shadow pictures.
This object is attained in accordance with the invention in that the hinged armature is supported at its radially outer end by means of a cylindrical pin for pivotal movement about the axis of the pin and that the cylindrical pin is snapped into support forks located on both sides of the radially outer yoke leg, which support forks are made of plastic and have fork legs enclosing between themselves a partially cylindrical support surface extending for more than 180.degree..
In the needle print head of the invention the hinged armature has a spatially fixed and definite pivot axis. The cylindrical pin can be made very precisely by simple means. The coengaging bearing surfaces for the pin which are formed on one hand by the pin and on the other hand by a receiving bearing part made of plastic are subject to small wear, which wear moreover occurs for all of the hinged armatures in at least nearly the same degree and in the same way. The elasticity of the plastic makes possible a snapping in of the pins. Therefore, the hinged armatures can be assembled without the need for great care. At the same time the plastic assures a small wear of the support pin. Moreover, the support forks can be made in a simple way and with high precision as injection molded parts. Preferably, a return spring engages the hinged armature at a point lying between the print needle and the radially inner yoke leg. Since in accordance with the invention the hinged armature is formed as a one-armed lever and the return spring engages the hinged armature at a point relatively far from the bearing axis, tolerances in the spring strength or in the spacing of the engagement point of the spring on the hinged armature from the bearing axis have little effect on the striking force of the print needle. Therefore, in the case of the arrangement of the invention the striking forces of different new hinged armatures lie in a relatively narrow tolerance range. Accordingly, the needle print head of the invention delivers a printed picture of very good quality. Because of the arrangement of the return spring relatively near to the print needle the rebound of the hinged armature on the stop is reduced, so that the danger of creating shadow pictures at high operating frequencies is avoided.
The fastening of the support forks to the magnet yoke can be accomplished in a simple way by having the support forks connected to one another through a metal bar which is fastened to the radially outer yoke leg as, for example, by spot welding.
The return spring is preferably a helical compression spring which on one side engages a plate carrying the magnet yoke and which on its other side engages the associated hinged armature, the helical compression spring advantageously being guided in a tube and, for reducing the friction between the spring and the hinged armature, carrying at its end facing the hinged armature a ball received in a corresponding recess in the hinged armature.
In order to permit adjustment of the excursion of the hinged armature it is advantageous if all of the hinged armatures are adjustable in the direction of movement of the print needles by a common stop. For dampening the rebound the stop can carry, on its side facing the hinged armatures, a layer of suitable dampening material.
Along with the already mentioned advantages the support for the hinged armature provided by the invention in comparison with a support of the hinged armature by a spring has the advantage that it allows a definite air gap between the hinged armature and the magnet yoke to be precisely maintained. In this way a sticking of the hinged armature to the magnet yoke can be dependably prevented.
Further features and advantages of the invention will be apparent from the following description, which in connection with the accompanying drawings explain the invention in connection with a preferred embodiment.